Plastic optical fiber preform, and process and apparatus for producing the same

ABSTRACT

A process for producing a preform for plastic optical fiber having a refractive index distribution in which the refractive index is gradually decreased from the center of the preform toward the outer periphery thereof, by depositing a deposition layer comprising a polymer A (refractive index: N a ) and a refractive index modifier having a refractive index different from that of the polymer A onto an inner surface of a hollow cylindrical member rotating about an axis thereof, by use of vapor-phase deposition based on a CVD (Chemical Vapor Deposition) process, or a coating process. A plastic optical fiber preform comprising deposition layers having a gradually increasing refractive index distribution is formed by changing the mixing ratio between the polymer A and the refractive index modifier constituting the deposition layer.

TECHNICAL FIELD

The present invention relates to a preform for plastic optical fiber, aprocess and an apparatus for producing a plastic optical fiber, and aprocess for the drawing of a plastic optical fiber.

BACKGROUND ART

A plastic optical fiber comprising a core and a cladding both of whichcomprise a plastic is rather suitable for an optical transmission lineto be used for a short distance such that the transmission loss in theline disposed between devices for sending and receiving an opticalsignal poses substantially no problem (for example, that betweenelectronic devices), as compared with a glass optical fiber. Inaddition, since a plastic optical fiber can generally be produced at alower cost than that of a glass optical fiber, such a fiber is widelyused as an optical transmission line for a short distance. Thus, theimportance of a plastic optical fiber has been increased, particularlyin view of a project or design for a next-generation communicationnetwork such as LAN (local area network) and ISDN (integrated servicedigital network).

Heretofore, there has been put into practical use a plastic opticalfiber 1, as shown in a schematic perspective view of FIG. 23, whichcomprises a core 2 comprising a resin such as PMMA (polymethylmethacrylate resin), PC (polycarbonate resin), or a copolymer thereof;and a cladding 3 comprising a fluorine-containing resin, etc., and has arefractive index distribution (profile) as shown in FIG. 24, i.e., astep-index (SI) type optical fiber.

In addition, as an optical fiber which is capable of transmitting alarger quantity of information per unit time than the above-mentionedSI-type optical fiber, there has been proposed a graded-index (GI) typeoptical fiber having a refractive index distribution as shown in FIG.25. Such a GI-type optical fiber is disclosed in Japanese PatentPublication (KOKOKU) Nos. 5857/1977 (Sho 52-5857) and 30301/1979 (Sho54-30301), and Japanese Laid-open Patent Application (KOKAI) Nos.130904/1986 (Sho 61-130904) and 162008/1986 (Sho 61-162008). However,these optical fibers still have various unsolved problems in view of theproduction thereof, etc., and an optical fiber having a desired propertyhas not been obtained yet.

More specifically, in the conventional plastic optical fibers, since therefractive index distribution has been formed by utilizing a specialchemical reaction such as one employing a difference in reactivity, andone employing a gel effect, there has been a severe limitation on theconditions such as the size of a preform, and the kind of a material, inview of the provision of a desired refractive index. Accordingly, theconventional plastic optical fibers have posed a problem such that massproduction thereof is difficult or a fiber material having excellenttransmission property and high reliability is difficult to be obtained.

Further, in the conventional optical fibers as described above, it isdifficult to freely control the reaction for providing the refractiveindex distribution, and therefore it has been difficult to obtain afiber having an ideal GI-type refractive index distribution in a goodyield.

Furthermore, Japanese Laid-Open Patent Application No. 16504/1990 (Hei2-16504) discloses a process for producing a plastic optical fiber byconcentrically extruding a laminate product of two or more species ofpolymerizable mixtures for providing mutually different indices.However, according to the present inventors' investigation, such aprocess has following problems.

More specifically, since the above process is a laminating-extrusionprocess and it can provide extrusion steps corresponding to only aboutten layers, the resultant product is inevitably caused to have astepped-type refractive index distribution. When an optical fiber havingsuch a stepped refractive index distribution is used, it is difficult totransmit a large quantity of information. Further, in the above process,it is suggested that a continuous and smooth refractive indexdistribution can be provided by further diffusing a monomer into theproduct after the laminating-extrusion process. However, when suchmonomer diffusion is employed, the number of the production steps isincreased and the resultant productivity becomes low. In addition, sincethe above process includes an operation for conducting monomer diffusionthe control of which is difficult, it is difficult to provide an idealGI-type refractive index distribution.

Further, there is proposed a process for producing a preform for plasticoptical fiber having a continuously changing refractive index byrepetitively pouring two species of materials having mutually differentrefractive index differences into a hollow cylindrical member andsubjecting such materials to polymerization and lamination under theaction of a centrifugal force (as described in Japanese Laid-Open PatentApplication No. 119509/1985 (Sho 60-119509)). However, it is notnecessarily easy in this process to regulate the resultant refractiveindex so as to provide a desired and designed value, and the productioncost tends to become somewhat higher.

In addition, Japanese Laid-Open Patent Application No. 124602/1993 (Hei4-124602) discloses a process for producing a plastic optical fiber bysubjecting a core material to fiber spinning so as to provide apredetermined diameter, and coating the resultant product with acladding material. However, in order to produce a GI-type plasticoptical fiber by such a coating operation using a cladding material, itis necessary to conduct multiple-stage coating operations, so that theproduction process becomes complicated.

On the other hand, it has also been proposed that a GI-type preform(base material) is prepared in advance and is subjected to hotstretching to provide a fiber (Polymer Preprints, Japan, vol. 41, No. 7,pp. 2942-2944, Autumn of 1992). It is conceivable that such a processcan reduce the number of production steps and can prepare variousspecies of fibers having different outer diameters. However, accordingto the present inventors' investigation, in such a process wherein theGI-type preform is simply inserted into a drawing furnace to besubjected to fiber drawing, a fluctuation in the outer diameter afterthe fiber drawing is liable to occur and the strength of the resultantfiber thus obtained tends to become lower than that of fiber produced byother process.

In general, in a plastic optical fiber to be actually used, a coverportion called as a jacket layer or a sheath layer is further providedon the above-described cladding layer so as to protect the main body ofthe plastic optical fiber (as disclosed in FIG. 1 of Japanese Laid-OpenPatent Application No. 230104/1985 (Sho 60-230104)). As disclosed inJapanese Laid-Open Patent Application Nos. 178302/1983 (Sho 58-178302)and 57811/1985 (Sho 60-57811), etc., a fiber having a good heatresistance and a good weathering resistance can be provided by selectinga jacket or sheath material having a good heat resistance and a goodweathering resistance.

Further, Japanese Laid-Open Patent Application No. 190204/1992 (Hei4-190204) discloses a technique such that an inorganic filler isincorporated into a jacket layer so as to facilitate the formation ofmetal plating on the surface of the jacket layer.

In these conventional methods, an optical fiber preform is drawn into afiber shape and then a cover portion is formed on the resultant fiber byutilizing a method such as die coating and extrusion, and therefore theformation of the cover portion is troublesome and the productivitybecomes low, whereby the resultant production cost becomes high.

It is also theoretically conceivable that, instead of the provision ofthe above-mentioned cover portion, the main body of the optical fiber iscaused to have a considerably large outer diameter so that the claddinglayer may also function as a protecting layer. In this case, however,there is posed a new problem such that the amount of a plastic materialto be used per unit length is increased. Further, since the plasticmaterial to be used for a plastic optical fiber is one having a highoptical transparency which has been produced so as to provide a highpurity through purification, and has a high production cost, an increasein the amount of such as plastic material to be used per unit lengthconstitutes a factor of an increase in the production cost.

An object of the present invention is to provide a plastic optical fiberor a preform therefor, and a process and an apparatus for producing sucha plastic optical fiber or a preform, which have solved theabove-mentioned problems encountered in the Background Art.

Another object of the present invention is to provide a process and anapparatus for producing a plastic optical fiber or a preform therefor,which has a desired refractive index distribution and is capable ofbeing produced easily.

A further object of the present invention is to provide a process and anapparatus for producing a plastic optical fiber or a preform therefor,which has a desired refractive index distribution and is capable ofbeing produced at a low cost.

A further object of the present invention is to provide a (drawing)process for producing a plastic optical fiber which can maintain asufficient mechanical strength and can assure a long-term reliabilityafter the fiber formation.

A further object of the present invention is to provide a (drawing)process for producing a plastic optical fiber which can suppress achange in the outer diameter thereof due to heat and can assure along-term reliability after the fiber formation.

A further object of the present invention is to provide a process forproducing a plastic optical fiber having a desired refractive indexdistribution and having a jacket layer which is capable of being easilyformed.

A further object of the present invention is to provide a process forproducing a plastic optical fiber having a desired refractive indexdistribution and having a jacket layer which has been reduced in theproduction cost.

DISCLOSURE OF INVENTION

As a result of earnest study, the present inventors have found that itis very effective in achieving the above object to form, on an innersurface of a hollow cylindrical member rotating on an axis, a laminateor multi-layer structure which comprises a layer including a polymer anda material having a refractive index different from that of the polymer(hereinafter, referred to as "refractive index modifier") and in whichthe ratio of the polymer to the refractive index modifier is changed.

The process for producing a plastic optical fiber preform according tothe present invention is based on the above discovery. More specificallythe present invention provides a process for producing a preform forplastic optical fiber having a refractive index distribution in whichthe refractive index is gradually decreased from the center of thepreform toward the outer periphery thereof, by depositing a depositionlayer comprising a polymer A (refractive index: N_(a)) and a refractiveindex modifier having a refractive index different from that of thepolymer A onto an inner surface of a hollow cylindrical member rotatingabout an axis thereof, by use of vapor-phase deposition based on a CVD(Chemical Vapor Deposition) process, wherein the mixing ratio betweenthe polymer A and the refractive index modifier constituting thedeposition layer is changed thereby to gradually increase the refractiveindex of the deposition layer.

In the above process for producing a preform for plastic optical fiberaccording to the present invention, it is preferred that the refractiveindex modifier comprises a refractive index modifier B having arefractive index (N_(b)) higher than that of the polymer A, and themixing ratio of the refractive index modifier B constituting thedeposition layer is gradually increased.

On the other hand, in the above process for producing a preform forplastic optical fiber according to the present invention, it is alsopossible that the refractive index modifier comprises a refractive indexmodifier C having a refractive index (N_(c)) lower than that of thepolymer A, and the mixing ratio of the refractive index modifier Cconstituting the deposition layer is gradually decreased.

The present invention also provides an apparatus for producing a preformfor plastic optical fiber by forming a deposition layer comprising apolymer A (refractive index: N_(a)) and a refractive index modifierhaving a refractive index different from that of the polymer A on aninner surface of a hollow cylindrical member, by use of vapor-phasedeposition based on a CVD process, the apparatus comprising:

a rotating device for supporting the hollow cylindrical member so as tobe rotatable about an axis thereof,

a supply pipe located at one end portion of the hollow cylindricalmember, for supplying vapor of a raw material for forming the depositionlayer to the inner surface of the hollow cylindrical member,

a heating device for heating the deposition layer deposited in theinside-of the hollow cylindrical member, and

raw material-supplying means for supplying the vapor of the raw materialfor forming the deposition layer to the supply pipe;

wherein the raw material-supplying means includes refractiveindex-regulating means for changing the mixing ratio of the refractiveindex modifier in the vapor of the raw material.

In the present invention, it is also possible to use a coating processinstead of the above CVD process.

More specifically, the present invention further provides a process forproducing a preform for plastic optical fiber having a refractive indexdistribution in which the refractive index is gradually decreased fromthe center of the preform toward the outer periphery thereof, bydepositing a deposition layer comprising a polymer A (refractive index:N_(a)) and a refractive index modifier having a refractive indexdifferent from that of the polymer A onto an inner surface of a hollowcylindrical member rotating about an axis thereof, by use of a coatingprocess,

wherein the mixing ratio between the polymer A and the refractive indexmodifier constituting the deposition layer is changed thereby togradually increase the refractive index of the deposition layer.

The present invention further provides an apparatus for producing apreform for plastic optical fiber by forming a deposition layercomprising a polymer A (refractive index: N_(a)) and a refractive indexmodifier having a refractive index different from that of the polymer Aon an inner surface of a hollow cylindrical member, by use of a coatingprocess, the apparatus comprising:

a rotating device for supporting the hollow cylindrical member so as tobe rotatable about an axis thereof,

a supply pipe located at one end portion of the hollow cylindricalmember, for supplying vapor of a raw material for forming the depositionlayer to the inner surface of the hollow cylindrical member,

a heating device for heating the deposition layer deposited in theinside of the hollow cylindrical member, and

raw material-supplying means for supplying the raw material for formingthe deposition layer to the supply pipe;

wherein the raw material-supplying means includes refractiveindex-regulating means for changing the mixing ratio of the refractiveindex modifier in the raw material.

In the above process for producing a preform for plastic optical fiberby use of a coating process according to the present invention, it ispreferred that an initial coating solution is prepared by using thepolymer A (refractive index: N_(a)) and a refractive index modifier Bhaving a refractive index (N_(b)) higher than that of the polymer A, andthe mixing ratio of the refractive index modifier B in the coatingsolution is gradually increased.

On the other hand, in the above process for producing a preform forplastic optical fiber according to the present invention, it is alsopossible that an initial coating solution is prepared by using thepolymer A (refractive index: N_(a)) and a refractive index modifier Chaving a refractive index (N_(c)) lower than that of the polymer A, andthe mixing ratio of the refractive index modifier C in the coatingsolution is gradually decreased.

In the process for producing an optical fiber preform according to thepresent invention as described above (by use of either a CVD process ora coating process), it is not necessary to utilize a special chemicalreaction in the production step for the preform, and therefore thedegree of freedom is markedly increased in the size of a preform to beprepared and/or the selection of kind of material to be used therefor.Consequently, according to the present invention, a preform having asize suitable for a production system may readily be produced whileusing a material capable of imparting an excellent property to theresultant optical fiber, and therefore it is easy to mass-produce apreform having a desired refractive index distribution and a desiredproperty.

Particularly, the present invention has an advantage such that anon-polymerizable material capable of imparting an excellenttransmission property may be selected as the above-mentioned refractiveindex modifier.

The present inventors have further found that in a process for drawing aGI-type preform into a fiber, the strength of the fiber is changeddepending on the orientation of the polymer constituting the fiber, andhave further found that it is very effective in solving the aboveproblem in the fiber strength to appropriately adjust a drawing tensionat the time of the drawing of the fiber.

Accordingly, the present invention further provides a process forforming a plastic optical fiber by drawing while melting an opticalfiber preform under heating; the optical fiber preform comprising, atleast, a core comprising an organic polymer and a cladding layercomprising an organic polymer and disposed on the outer circumference ofthe core;

wherein the drawing is conducted under a drawing tension of 10 g or moreuntil winding-up of the optical fiber.

In addition, according to the present inventors' investigation, it hasbeen found that in a process for drawing a GI-type preform into a fiber,a fluctuation in the outer diameter of the resultant fiber after thecompletion of the drawing is occured depending on the orientation of thepolymer constituting the cladding of the fiber, and that it is veryeffective in solving the above problem of the fluctuation in the outerdiameter of the fiber after the drawing to adjust a drawing tensionappropriately at the time of the drawing of the fiber.

Accordingly, the present invention further provides a process forforming a plastic optical fiber by drawing while melting an opticalfiber preform under heating; the optical fiber preform comprising, atleast, a core comprising an organic polymer and a cladding layercomprising an organic polymer and disposed on the outer circumference ofthe core;

wherein the drawing is conducted under a drawing tension of 100 g orless until winding-up of the optical fiber.

In addition, the present invention further provides a preformfor-plastic optical fiber, comprising:

a core comprising an organic polymer,

a cladding layer comprising an organic polymer and disposed on the outercircumference of the core, and

a jacket layer disposed on the outer circumference of the claddinglayer;

wherein the jacket layer comprises a material which has the same qualityas the material constituting the cladding layer, and has a purity lowerthan that of the material constituting the cladding layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing an embodiment of theapparatus for producing an optical fiber preform by utilizing depositionbased on an inside-surface CVD process in the present invention.

FIG. 2 is a schematic perspective view showing an example of preformprovided by the deposition shown in FIG. 1.

FIG. 3 is a schematic view showing a refractive index distribution ofthe preform shown in FIG. 2.

FIG. 4 is a schematic perspective view showing a state of a preformwhich has been formed by the deposition and then collapsed.

FIG. 5 is a schematic view showing a refractive index distribution ,atthe state as shown in FIG. 4.

FIG. 6 is a schematic perspective view showing another embodiment of theapparatus for producing an optical fiber preform by utilizing aninside-surface CVD process in the present invention.

FIG. 7 is a schematic perspective view showing an embodiment of theapparatus for producing an optical fiber preform by utilizing depositionbased on an inside-surface coating process in the present invention.

FIG. 8 is a schematic perspective view showing another embodiment of theapparatus for producing an optical fiber preform by utilizing aninside-surface coating process in the present invention.

FIG. 9 is a schematic sectional view showing an embodiment of thestructure of a fiber drawing furnace for drawing an optical fiberpreform in the process according to the present invention.

FIG. 10 is a schematic perspective view showing an example of thestructure of an optical fiber having a jacket layer.

FIG. 11 is a schematic cross-sectional view showing an example of thestructure of an optical fiber having a jacket layer.

FIG. 12 is a schematic cross-sectional view showing an example ofoptical transmission in the optical fiber shown in FIG. 11.

FIG. 13 is a schematic cross-sectional view showing an example of thestructure of an optical fiber not having a jacket layer.

FIG. 14 is a schematic cross-sectional view showing an example ofoptical transmission in the optical fiber shown in FIG. 13.

FIG. 15 is a schematic perspective view showing a relationship among acladding layer (layer thickness: D₁), a core (diameter: D₂), and ajacket layer (layer thickness: D₃).

FIG. 16 is a schematic view showing an embodiment of GI-type refractiveindex distribution of the optical fiber shown in FIG. 15.

FIG. 17 is a schematic perspective view for illustrating a step(immersing step) in a case where an optical fiber preform having ajacket layer is prepared by a coating method using casting.

FIG. 18 is a schematic perspective view for illustrating a step(pulling-up step) in a case where an optical fiber preform having ajacket layer is prepared by a coating method using casting.

FIG. 19 is a schematic perspective view for illustrating a step (dryingstep) in a case where an optical fiber preform having a jacket layer isprepared by a coating method using casting.

FIG. 20 is a schematic perspective view for illustrating a step (step ofadding a refractive index modifier) in a case where an optical fiberpreform having a jacket layer is prepared by a coating method usingcasting.

FIG. 21 is a graph showing a refractive index distribution of each ofthe optical fibers obtained in Examples 3 and 6.

FIG. 22 is a graph showing a relationship between fiber drawing tensionand transmission loss in each of the optical fibers obtained in Examples10-13 and Comparative Examples 5-7.

FIG. 23 is a schematic perspective view showing an example of thestructure of a conventional plastic optical fiber preform.

FIG. 24 is a schematic view showing an SI-type refractive indexdistribution.

FIG. 25 is a schematic view showing a GI-type refractive indexdistribution.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in detail withreference to the accompanying drawings as desired.

(Polymer)

In the present invention, as the polymer A (refractive index: N_(a))constituting a deposition layer to be deposited on an inside surface ofa hollow cylindrical member rotating about an axis, a known transparentpolymer may be used without particular limitation. Specific example ofsuch a polymer may include: homopolymers of methyl methacrylate(polymethyl methacrylate: PMMA), polycarbonate (PC); or a transparentcopolymer of methyl methacrylate and another monomer. Preferred examplesof such an "another monomer" may include: acrylic monomers such asmonofunctional (meth)acrylates, fluorine-containing or fluorinated alkyl(meth)acrylates, polyfunctional (meth)acrylates, polyfunctional(meth)acrylates, acrylic acid, and methacrylic acid; and styrene-typemonomers such as styrene and chlorostyrene.

Among the polymers as described above, polymethyl methacrylate(refractive index n=1.490) or polycarbonate (n=1.59) may particularlypreferably be used.

(Refractive index modifier)

In the present invention, as the material (refractive index modifier)having a refractive index different from that (N_(a)) of the abovepolymer A, either of a refractive index modifier having a refractiveindex (N_(b)) higher than that of the polymer (refractive index modifierB), and a refractive index modifier having a refractive index (N_(c))lower than that of the polymer (refractive index modifier C) may beused.

The molecular weight of the refractive index modifier is notparticularly restricted, as long as it may provide a desired refractiveindex distribution and may be stably copresent with the above-mentionedpolymer. In addition, the refractive index modifier per se may have apolymerizable functional group (for example, an unsaturatedpolymerizable group such as vinyl group CH₂ ═CH--). In other words, therefractive index modifier may be a monomer or a mixture thereof, or anoligomer or polymer.

The absolute value of a difference between the refractive index N_(a) ofthe polymer A and the refractive index (N_(b) or N_(c)) of the abovemodifier, |N_(a) -N_(b) | or |N_(a) -N_(c) |, may preferably be 0.01 ormore, more preferably 0.02 or more (particularly preferably, 0.03 ormore).

In the present invention, when the above polymer A comprises polymethylmethacrylate (PMMA) (N_(a) =n=1.49), preferred examples of therefractive index modifier to be used in combination with the polymer Amay include: as the modifier B having a higher refractive index, butylbenzyl phthalate (N_(b) =n=1.536), 2-phenylethyl acetate (n=1.51),dimethyl phthalate (n=1.515), diphenyl sulfide (n=1.635), vinyl benzoate(n=1.577), benzyl methacrylate (n=1.568), and diallyl phthalate(n=1.518), etc. Among the above specific examples, vinyl benzoate,benzyl methacrylate, and diallyl phthalate are refractive indexmodifiers having a polymerizable functional group.

On the other hand, specific examples of the refractive index modifier Chaving a lower refractive index may include: hexyl acetate (N_(c)=n=1.408), bis(3,5,5-trimethylhexyl) phthalate (n=1.487), andbis(2-methylhexyl) phthalate (n=1.486), etc.

(Deposition method)

In the present invention, as the process for forming a multi-layerstructure (inside-surface deposition) comprising a layer wherein theratio of the above polymer A to the refractive index modifier ischanged, on an inner surface of a hollow cylindrical member rotatingabout an axis, either of a vapor deposition process and a coatingprocess may be used. In an embodiment using the vapor depositionprocess, a chemical vapor deposition (CVD) process may preferably beused.

(Production of preform by CVD)

Hereinbelow, there is described an embodiment of the present inventionfor producing a preform for plastic optical fiber by using aninside-surface CVD process with reference to FIG. 1.

FIG. 1 is a schematic perspective view showing an embodiment of theapparatus to be usable in the production of an optical fiber preformaccording to the present invention (an apparatus for producing anoptical fiber preform according to the present invention).

Referring to FIG. 1, this apparatus is one for producing a plasticoptical fiber preform by depositing vapor of a raw material in theinside of a hollow cylindrical member through a CVD process. Thisapparatus comprises: a rotating device (not shown) for supporting ahollow cylindrical member 11 so as to be rotatable around an axisthereof; a supply pipe 12 (in the form of a nozzle in FIG. 1) which islocated at the axis center of the hollow cylindrical member, isreciprocatingly movable in the direction of the above axis, andcomprises a plurality of spraying ports so as to spray the vapor of theraw material supplied thereto onto the inner surface of the hollowcylindrical member 11; a heating device 13 (in the form of a ring heaterin FIG. 1) for conducting a heat treatment of an organic raw materialdeposited on the inner surface of the hollow cylindrical member 11,which is located so as to be reciprocatingly movable along the axisdirection; and a raw material-supplying means for supplying vapor as araw material for the formation of a deposition layer (in the form ofvapor of an organic raw material in FIG. 1). In the embodiment shown inFIG. 1, the raw material supplying means comprises a supply vessel 14(in the form of a supply tank in FIG. 1) and a vessel 15 for arefractive index modifier (in the form of a refractive index modifiertank in FIG. 1) for supplying the refractive index modifier to thesupply vessel 14.

In the embodiment of FIG. 1, the raw material for a polymer to besupplied to the above nozzle 12, is stored as an organic raw material 17in the supply tank 14 provided with a heating means 16 together with asolvent, and an inert gas 18 (such as N₂, Ar and He) is introduced intothe supply tank 14 so as to supply the vapor of the above material tothe nozzle 12.

On the other hand, a refractive index modifier B is stored in therefractive index modifier tank 15, and the refractive index modifierhaving a different refractive index is appropriately supplied to thesupply tank 14 from the refractive index modifier tank 15 so as tocontrol the resultant refractive index. More specifically, for example,as the deposition of a deposition layer comprising the above-mentionedpolymer A and refractive index modifier B is repeated on the innersurface of the hollow cylindrical member 11, the refractive indexmodifier B is introduced into the supply tank 14 from the refractiveindex modifier tank 15, whereby the refractive index in the supply tank14 is gradually changed to provide a predetermined gradient in theoptical refractive index (refractive index distribution).

In the embodiment of FIG. 1, when a refractive index modifier C having arefractive index lower than that of the polymer A is used, for example,a predetermined mixture comprising the polymer A and the refractiveindex modifier C (a mixture having a refractive index lower than that ofthe polymer A per se) is placed in the supply tank 14, and the polymer Astored in the refractive index modifier tank 15 is gradually supplied tothe supply tank 14 thereby to provide a desired refractive indexdistribution.

In the production apparatus as described above, the heating means 16 isnot necessarily required. In other words, the heating means 16 may beomitted depending on the kind of the organic material to be used incombination with the apparatus.

Referring to FIG. 1, the polymer A has been poured into the supply tank14 as an organic raw material solution 17 together with a solvent whichis capable of dissolving the polymer A. As the deposition of adeposition layer comprising the polymer A and the refractive indexmodifier B is repeated on the inner surface of the hollow cylindricalmember 11, the refractive index in the supply tank 14 is graduallychanged by introducing the refractive index modifier B from therefractive index modifier tank 15 to the supply tank 14, whereby apredetermined gradient in the optical refractive index (refractive indexdistribution) is provided.

In addition, as shown in FIG. 1, the nozzle 12 is moved reciprocatinglyin the direction of an axis while the hollow cylindrical member 11 isrotated, and the vapor of the raw material is sprayed onto the innersurface of the hollow cylindrical member 11 and heating is conducted byusing the heating means 13, thereby to deposit the raw material having agradually changing refractive index on the inner surface of the hollowcylindrical member 11. Through the above procedure, there is formed acore layer wherein the refractive index is gradually decreased in theradial direction of from the center of the resultant preform toward theouter periphery thereof.

After the CVD deposition as described above is completed, as shown in aschematic perspective view of FIG. 2, there is provided a preform 20having a cavity portion 20a extending in the direction of an axis, whichcorresponds to the nozzle 12 extending along the center axis. FIG. 3schematically shows a profile of a refractive index distribution of theresultant preform before the collapse thereof.

The preform 20 thus prepared is collapsed by melting under heating so asto fill up the above cavity 20a (FIG. 2), whereby a plastic opticalfiber preform 21 as shown in a schematic perspective view of FIG. 4 isobtained. FIG. 5 is a graph schematically showing a refractive indexdistribution of the. GI-type optical fiber preform thus obtained.

A desired plastic optical fiber may be obtained by subjecting theoptical fiber preform 21 thus obtained to an ordinary fiber drawingprocedure. In such a drawing procedure, a plastic fiber may for examplebe obtained by drawing the above fiber preform 21 under melting due toheating, while the fiber preform 21 is kept vertically.

In the present invention, a substance which is polymerizable under theaction of energy such as light (for example, benzoin, benzoin methylether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutylether, 2-methylbenzoin, benzil, benzil dimethyl ketal, benzil diethylketal, etc.) may be used as at least one of the raw material for theabove polymer A (such as monomer) and the refractive index modifier B,and it may be subjected to polymerization (for example,photo-polymerization) under the application of energy ray such asultraviolet ray, to fix the predetermined refractive index distributionas described above. In such an embodiment, it is particularly preferredto use an initiator for the photo-polymerization wherein cleavage mayoccur between different species of atoms due to light energy (the energyrequired for such cleavage may be smaller than that required for thecleavage of a single bond between atoms of the same kind). Specificexamples of an initiator wherein cleavage may occur between differentspecies of atoms due to light energy may include those as describedbelow.

<Initiators wherein cleavage may occur at a single bond betweendifferent species of atoms; carbon and phosphorus atoms>

benzoyl diphenylphosphine oxide

benzoyl dimethylphosphine oxide

benzoyl diethylphosphine oxide

2-methylbenzoyl diphenylphosphine oxide

2-methylbenzoyl dimethylphosphine oxide

2-methylbenzoyl diethylphosphine oxide

2,4-dimethylbenzoyl diphenylphosphine oxide

2,4-dimethylbenzoyl dimethyiphosphine oxide

2,4-dimethylbenzoyl diethylphosphine oxide

2,4,6-trimethylbenzoyl diphenylphosphine oxide

2,4,6-trimethylbenzoyl dimethylphosphine oxide

2,4,6-trimethylbenzoyl diethylphosphine oxide

<Initiators wherein cleavage may occur at a single bond betweendifferent species of atoms; carbon and sulfur atoms>

benzoyl phenyl sulfide

2-methylbenzoyl phenyl sulfide

4-methylbenzoyl phenyl sulfide

2,4-dimethylbenzoyl phenyl sulfide

2,4,6-trimethylbenzoyl phenyl sulfide

4-chlorobenzoyl phenyl sulfide

benzoyl 2-methylphenyl sulfide

benzoyl 4-methylphenyl sulfide

benzoyl 2,4-dimethylphenyl sulfide

benzoyl 2,4,6-trimethylphenyl sulfide

benzoyl 4-chlorophenyl sulfide

In a case where such a photo-polymerization is effected, atime-dependent change in the refractive index distribution based onheat, etc. may further be suppressed, thereby to provide an opticalfiber which can particularly suitably be used in a region with a hightemperature or considerable heating.

Further, in such an embodiment utilizing photo-polymerization, it isalso possible to constitute the hollow cylindrical member 11 by use of amaterial capable of transmitting ultraviolet light and to supplyultraviolet light from the outside of the hollow cylindrical member 11.Such an embodiment wherein ultraviolet light is supplied from theoutside of the hollow cylindrical member 11 is preferred in view ofefficiency in the polymerization.

FIG. 6 is a schematic perspective view showing another embodiment of anapparatus (for inside-surface CVD process) for producing an opticalfiber preform according to the present invention.

The embodiment shown in FIG. 6 has the same structure as that of theembodiment shown in FIG. 1 except that the nozzle 12 in FIG. 1 isomitted and a supply pipe 19 for introducing the vapor of a raw materialis directly connected to an end portion of the rotating hollowcylindrical member 11 so as to directly supply the vapor of the rawmaterial into the hollow cylindrical member 11.

(Production of preform by coating method)

Next, there is described an embodiment for producing a preform forplastic optical fiber by utilizing an inside-surface coating process inthe present invention with reference to FIG. 7.

FIG. 7 is a schematic perspective view showing an embodiment of anapparatus usable for the production of an optical fiber preformaccording to the present invention (an apparatus for producing anoptical fiber preform according to the present invention).

Referring to FIG. 7, this apparatus comprises: a rotating device (notshown) for supporting a hollow cylindrical member 31 so as to berotatable about an axis thereof; a supply pipe (in the form of aspraying nozzle in FIG. 7) 32 which is located at the axis center of theabove hollow cylindrical member 31 and is movable in the direction ofthe axis thereof so as to apply an organic material onto the innersurface of the hollow cylindrical member 31; a drying device (in theform of a ring heater in FIG. 7) 33 for heating the organic materialdeposited on the inside of the hollow cylindrical member 31 to remove asolvent contained therein; a supply tank 34 for supplying the organicmaterial; and a tank 35 for a refractive index modifier.

Hereinbelow, there is described an embodiment of the process forproducing a preform for plastic optical fiber wherein the apparatus inFIG. 7 is used.

Referring to FIG. 7, the polymer A has been poured as a spraying rawmaterial into the supply tank 34 together with a solvent which iscapable of dissolving the polymer A, while a refractive index modifier Bis stored in the refractive index modifier tank 35. As the spraying ontothe inner surface of the hollow cylindrical member 31 is repeated, therefractive index modifier B is supplied from the refractive indexmodifier tank 35 to the supply tank 34 to gradually change therefractive index in the supply tank 34 so as to obtain a predeterminedgradient in the optical refractive index (refractive indexdistribution).

In addition, as shown in FIG. 7, while the hollow cylindrical member 31is rotated, the nozzle 32 is moved reciprocatingly in the direction ofthe axis of the hollow cylindrical member 31, and a raw material(composition) comprising the polymer A and the refractive index modifierB is sprayed under drying based on a dryer 33. By use of the aboveprocedure, the spraying raw material having a gradually changingrefractive index is sprayed thereby to form a core layer in which therefractive index is gradually decreased in the radial direction from thecenter of the preform toward the outer periphery thereof.

After the completion of the spraying, as shown in FIG. 2 describedabove, a desired GI-type plastic optical fiber preform is provided bysubjecting the resultant product to melting under heating so as to becollapsed. The optical fiber preform thus obtained is then subjected toan ordinary fiber drawing procedure, in the same manner as in the aboveembodiment using the CVD process. For example, a plastic optical fibermay be obtained by subjecting the above optical fiber preform to meltingunder heating, while the preform is kept vertically.

FIG. 8 shows another embodiment of the apparatus for producing anoptical fiber preform (by a coating process) according to the presentinvention.

The embodiment shown in FIG. 8 has the same structure as that of theembodiment shown in FIG. 7, except that, instead of the nozzle 32movable in the direction of an axis as in FIG. 7, a nozzle 32a having aplurality of spraying ports 38 disposed along the direction of the axisof the hollow cylindrical member 31 is used, and a desired coating layermay be deposited onto an inner surface of the hollow cylindrical member31 without moving the nozzle 32a in the axis direction.

(Fiber drawing process)

Next, there is described a process for drawing an optical fiber preforminto an optical fiber with reference to a schematic cross sectional viewof FIG. 9. FIG. 9 is a schematic sectional view showing an apparatus forforming a plastic optical fiber by fiber drawing.

Referring to FIG. 9, a heater 42 and a core tube 43 are provided in theinside of the main body 41 of a fiber drawing furnace. When such a fiberdrawing apparatus is used, a resin base material (preform) 40 foroptical fiber is inserted into an upper opening 41a in the furnace mainbody 41, and melted under heating in the heating furnace 41, to be drawnor spun into a plastic optical fiber 46 having a predetermined outerdiameter. The plastic optical fiber 46 thus drawn is pulled out througha lower opening 41b of the furnace 41, and then the plastic opticalfiber 46 is wound up by use of a winding-up device 45 while the outerdiameter of the resultant optical fiber 46 is measured by means of anouter diameter-measuring device (monitor) 44.

As the above-mentioned preform 40, one having a GI-type refractive indexdistribution in the core and cladding layer may preferably be used. Insuch a fiber drawing method, for example, it is preferred to use apreform 40 which has been prepared by using a polymethyl methacrylate(PMMA) having excellent optical transparency for a cladding and using acompound having a higher refractive index for the core. In this case,the compound having a higher refractive index to be added to the core,and the outer diameter or the length of the preform are not particularlyrestricted.

It is preferred to apply a drawing tension of 10 g or more to thepreform (base material) 40 comprising a polymer during a period of timefrom the heating thereof based on the heating furnace 41 to thewinding-up thereof by the winding-up device 45. When the degree oforientation in a polymer is low, the molecules constituting theresultant polymer product assumes a randomly-oriented structure, andtherefore the strength thereof is weak under stretching. In contrastthereto, according to the present inventors' knowledge, when the drawingtension is 10 g or more, the molecules are oriented along thelongitudinal direction of the resultant fiber, whereby the tensilestrength of the fiber may be improved and the long-term reliabilitythereof may be assured.

On the other hand, the above drawing tension may preferably be 100 g orless. When the drawing tension exceeds 100 g, the resultant fiber isliable to cause shrinkage under the action of heat. According to thepresent inventors' knowledge, the reason for the occurrence of such aphenomenon may be considered as follows:

Thus, when the polymer constituting the cladding layer of an opticalfiber is considerably oriented under the action of a drawing tensionlarger than 100 g, the fiber will be wound up around a reel, etc., insuch a state that the cladding is so oriented. As a result, when thepolymer constituting the cladding is supplied with heat, it is presumedthat shrinkage occurs on the basis of the recovery of the polymer to itsoriginal state.

In a case where the cladding is shrunk in such a manner, according tothe present inventors' knowledge, it is presumed that the optical fiberper se is also shrunk and a strain is applied to the optical fiber inthe longitudinal direction thereof to cause a structure defect of thefiber per se thereby to increase the transmission loss.

Accordingly, in the present invention, it is preferred to employ adrawing tension of 100 g or less to prevent the considerable orientationof the polymer constituting the cladding.

According to the present inventors' knowledge, when the above-mentionedpreform 40 is melted under heating to be spun, the outer diameter of theoptical fiber 46 may preferably be 1000 μm or less. In a case where theouter diameter exceeds 1000 μm, a decrease in the strength is notobserved even when the degree of orientation of molecules is low. On theother hand, in a case where the outer diameter is 1000 μm or less, adecrease in the strength is considerable when the degree of orientationof molecules is low. However, as described above, in a case where thedrawing tension is set to be 10 g or more, the tensile strength of thefiber 46 is improved on the basis of the orientation of molecules alongthe longitudinal direction of the fiber thereby to assure long-termreliability, even when the outer diameter of optical fiber 46 is set to1000 μm or less.

(Optical fiber preform)

The plastic optical fiber preform obtained by the production processaccording to the present invention as described above generally has arod-like shape comprising a core and a cladding layer. In the presentinvention, it is preferred that a jacket layer is further formed asdesired on the outer circumference of the rod comprising the core andthe cladding layer, and the jacket layer comprises a material havingsubstantially the same quality as that of an organic polymerconstituting the cladding layer and having a lower purity than that ofthe organic polymer constituting the cladding layer.

In such an embodiment, the material constituting the jacket layer maypreferably be such a polymer B₁ which has the same quality as that ofthe polymer B constituting the cladding layer and has a lower puritythan that of the polymer B. Herein, "the same quality" means that themonomer constituting the material for the jacket layer is substantiallycommon with the monomer constituting the polymer B for the claddinglayer. More specifically, at least 90 mol % (more preferably, at least95 mol %) of the monomer constituting the jacket layer is the same asthe monomer constituting the polymer B for the cladding layer.

In the case of a copolymer, when the common monomers in the jacket layerand the cladding layer are respectively denoted by M₁ and M₂, themonomer constitution of the cladding layer is denoted by {M₁ (a₁ mol)+M₂(b₁ mol)}, and the monomer constitution of the jacket layer is denotedby {M₁ (a₂ mol)+M₂ (b₂ mol)} (provided that a₁ ≦a₂ and b₁ ≦b₂), thevalue of (a₂ +b₂)/(a₁ +b₁) may preferably be at least 90 mol % (morepreferably, at least 95 mol %).

Since the polymer B constituting the cladding layer has a high purityand contributes to light transmission, a polymer having a purity of atleast 99 may preferably be used as the polymer B constituting thecladding layer. On the other hand, since the polymer B₁ constituting thejacket layer does not directly contribute to light transmission, apolymer having a purity of not more than 99% may preferably be used inview of cost, etc. The polymer B₁ may preferably have a purity of atleast about 80% (more preferably, at least about 90%) in view of theadhesive property thereof with the cladding. However, this "purity" isused in a manner such that an impurity not contributing to lighttransmission at all (polymerization inhibitor, etc.) is excluded fromthe consideration of the purity. Accordingly, a component whichcontributes to light transmission (for example, a material to be addedso as to change the refractive index) is not considered as an "impurity"relating to the evaluation of the above-mentioned purity.

As described above, in a case where a material having a low purity isused for the jacket layer, a material having a low cost (for example, acommercially available and inexpensive polymer) can be used for thejacket layer, and therefore a plastic optical fiber having a low costbut having a good transmission property may be provided even when theouter diameter of the fiber is increased.

In the present invention, it is further preferred that the materialconstituting the jacket layer is one having the same quality as both ofthe core and the cladding, in consideration of the adhesion between thecladding and the core. More specifically, in such an embodiment, amaterial comprising at least 90% (more preferably, at least 95%) of thecomponent (monomer, etc.) for the core material may preferably be usedas the material constituting the jacket layer. On the other hand, inview of the production cost, a material comprising 99% or less of thecomponent (monomer, etc.) for the core material may preferably be usedas the material constituting the jacket layer.

Here, when a material having the same quality as those for the core andcladding layer is used as the material constituting the jacket layer, ascompared with a case wherein the jacket layer is formed by using anothermaterial of different quality, the handling property at a working site,etc., may be improved and simultaneous peeling of the jacket layer maybe effectively prevented in the peeling-off of a resin layer (anoutermost layer 104 in FIG. 10 as described below).

The material constituting the jacket layer may preferably have a meltingpoint which is substantially equal to that of the core material. Morespecifically, when the melting point of the material for the jacketlayer is denoted by m_(j) and the melting point of the core material isdenoted by m_(c), |m_(j) -m_(c) | (absolute value) may preferably be notmore than 10° C. (i.e., the value of (m_(j) -m_(c)) may preferably bewithin ±10° C.).

When a material having a melting point substantially different from thatof the core material is used as the material constituting the jacketlayer, the cladding layer and the jacket layer are liable to be meltednonuniformly (to be melted separately) at the time of the melting of thepreform during a fiber drawing operation.

FIG. 10 is a schematic perspective view showing an embodiment of thestructure of an optical fiber which has been formed by forming apreform, coating the preform with a resin layer, and further subjectingthe resultant preform to fiber drawing. Referring to FIG. 10, theoptical fiber in this embodiment comprises a core 101, a cladding layer102 comprising a high-purity polymer and located on the outercircumference of the core 101, and a jacket layer 103 comprising alow-purity polymer and located on the outer circumference of thecladding layer 102.

When the jacket layer 102 is formed on the circumference of the claddinglayer in the state of the preform in the above-mentioned manner, thejacket layer is simultaneously formed in the melt-drawing of thepreform. Accordingly, in such an embodiment, it is not necessary to formthe jacket layer by coating after the drawing, and therefore theresultant productivity is markedly improved.

Further, an outermost layer of resin layer 104 may be formed as desiredon the circumference of the jacket layer 103, for the purpose ofdiscriminations etc. In such a case, in the present invention, it is notnecessarily required to impart a function as a protecting layer to theresin layer 104, and therefore a resin which is more inexpensive ascompared with that in a conventional case can be used as the materialfor the resin layer 104. In addition, there is a further advantage thatthe resin layer 104 can be made thinner.

In general, when a connector is mounted to an optical fiber for thepurpose of connection, it is necessary to remove the resin layer 104from the plastic optical fiber. However, when the fiber per se has thejacket layer 103 as in the plastic optical fiber produced according tothe present invention, the fiber can maintain sufficient strength evenafter the removal of the resin layer 104 therefrom, and therefore such afiber has an advantage that good strength may be maintained even in theconnector portion.

In general, connecting means such as connectors are standardized in mostcases. In the case of a conventional optical fiber, the main body outerdiameter of which has been simply increased, it is required that eventhe neighborhood of the outer periphery of the fiber not contributing totransmission is constituted by using a material having a high purity,for the purpose of meeting the standard, and therefore the resultantproduction cost is markedly increased. In contrast thereto, according tothe present invention, the production cost can be reduced by forming thejacket layer 103 having a low purity.

The process according to the present invention is particularly suitablefor a case in which the plastic optical fiber to be produced is aGI-type plastic optical fiber.

More specifically, in the production of an SI-type plastic opticalfiber, the fiber is generally produced by pull down a liquefied corematerial due to melting in most cases. On the other hand, generally inthe production of a GI-type plastic optical fiber, a preform having arefractive index distribution is first prepared and the preform is drawninto a fiber, as disclosed in Japanese Patent Publication No. 5857/1977(Sho 52-5857). Accordingly, the present invention has an advantage thatthe production of the a preform with a jacket is facilitated byconducting a jacketting step in series at the time of the formation ofthe GI-type preform.

It is also possible to incorporate a functional material having each ofvarious functions, such as anti-oxidant, light absorbing agent andlight-scattering agent, so as to cause the functional material toexhibit its function. These functional materials can be added to thejacket layer singly or in combination of at least two species thereof.

More specifically, for example, when an anti-oxidant (such as hinderedphenol, hindered amine, aryl amine, phosphite, and thio ether) isincorporated into the jacket layer, it is possible to cause such acomponent to exhibit a function of preventing opacification and ofpreventing coloring.

Further, as shown in FIGS. 11 and 12, when a light absorbing agent 105(such as benzotriazole, benzophenone, benzoate, and cyanoacrylate) isincorporated into the jacket layer 103 to be formed on the circumferenceof the core 101 and the cladding layer 102, stray light L_(x) (FIG. 14),which can propagate in the cladding layer 102 when propagating light Lis incident to an optical fiber having the structure as shown in FIG.13, can be effectively absorbed.

Further, a light-scattering agent such as TiO₂ powder may beincorporated into the jacket layer 103 so as to exhibit itslight-scattering function.

As shown in FIG. 15, the thickness D₁ of the cladding layer in theplastic optical fiber preform to be produced according to the presentinvention may preferably be not less than 10% and not more than 40% ofthe diameter D₂ of the core. Further, the thickness D₃ of the jacketlayer may preferably be greater than the thickness D₁ of the claddinglayer. When D₁ is less than 10% of D₂, a part of light for communicationalso propagates in the jacket layer having a low transparency, so thatthe transmission performance can be lowered. Most of light forcommunication propagates in the core but a part of the light forcommunication spreads to the outside of the core so as to propagate insuch a portion. Accordingly, the cladding may preferably has a thicknesslarger than that of the core to a certain extent.

On the other hand, when D₁ exceeds 40% of D₂, an expensive material forcladding is also used for a region which does not contribute to thepropagation of communication light at all, and therefore such a case isnot preferred in view of the production cost.

In the optical fiber preform provided with the jacket layer in such amanner, it is preferred that a GI-type refractive index distribution isformed so that the refractive index is gradually decreased from thecenter of the optical fiber preform toward the outer periphery thereof,as shown in FIG. 16. The process for producing such an optical fiberpreform is not particularly restricted, but it is preferred to use aninside-surface CVD process or an inside-surface coating process (such asspray coating process) as described above may preferably be used. When acoating process is used, a drying operation may be conductedsimultaneously with the coating operation, or the coating and dryingoperations may be conducted alternately.

Then, the optical fiber preform thus obtained may be subjected to anordinary fiber drawing procedure (e.g., an operation wherein the opticalfiber preform is melted under heating-while the preform is verticallymaintained), thereby to obtain a desired plastic optical fiber.

There is described an embodiment of the process for producing theabove-mentioned preform for plastic optical fiber (provided with ajacket layer) with reference to schematic perspective views of FIGS.17-20.

FIGS. 17-20 schematically show an example of the coating process (castcoating process), which is different from spray coating or brushcoating. In FIGS. 17-20, reference numeral 51 denotes a starting rod,numeral 52 denotes a coating tank, numeral 53 denotes a dryer, andnumeral 54 denotes a pipe for supplying a coating solution,respectively.

Referring to FIG. 17, the starting rod 51 is immersed in the coatingtank 52 and then pulled up (FIG. 17 and FIG. 18), and thereafter theresultant product is subjected to drying by use of the dryer 53 (FIG.19). Then, the above steps are treated or considered as one cycle, andwhile supplying a refractive index-adjusting raw material through thecoating solution-supplying pipe 54 for every cycle so as to gradually orsequentially change the concentration of a refractive index-adjustingraw material in the coating tank 52, such a casting operation isrepeated, thereby to form a cladding layer. Then, the raw material to beused is replaced with one having a lower purity, and a jacket layer isformed by using such a material. Through such a procedure, there isformed a preform for plastic optical fiber having a jacket layer andhaving a GI-type distribution as shown in FIG. 16 wherein the refractiveindex is gradually decreased from the center of the preform toward theouter periphery thereof.

Hereinbelow, the present invention will be described in further detail.

EXAMPLE 1

Referring to FIG. 1, polymethyl methacrylate (PMMA; refractive indexN_(a) =1.490) was used as a transparent polymer A, and a hollowcylindrical member 11 comprising the polymer A as a main component wasformed.

The above polymer A (PMMA) was dissolved in a solvent (tetrahydrofuran:THF) in a predetermined ratio (concentration: about 30 wt. %) and theresultant solution was poured into a supply tank 14. On the other hand,butyl benzyl phthalate ester as a refractive index modifier B having arefractive index (refractive index N_(b) =1.536) higher than that of thepolymer A was dissolved in THF (concentration: about 30 wt. %) to obtaina solution, and the resultant solution was placed in a refractive indexmodifier tank 15.

The hollow cylindrical member 11 obtained by the above-mentionedprocedure was mounted to an unshown rotating device and the vapor of araw material was uniformly supplied from the supply tank 14 onto theinner surface of the cylindrical member along the axis directionthereof, by use of a nozzle 12.

In the above-mentioned supply of the vapor, the refractive indexmodifier B was gradually fed (flow rate: about 10 ml/min) from therefractive index modifier tank 15 into the supply tank 14, so that themixing ratio of the refractive index modifier B was increased at everyvapor-supplying operation to sequentially increase the refractive indexof the deposition layer based on the vapor. As a result, there wasobtained a plastic optical fiber preform 21 (FIG. 4) having a GI-typerefractive index distribution in which the refractive index wasgradually decreased from the center of the preform toward the outerperiphery thereof (as shown in FIG. 5).

EXAMPLE 2

Referring to FIG. 6, polymethyl methacrylate (PMMA; refractive indexN_(a) =1.492) was used as a transparent polymer A and hexyl acetatehaving a refractive index (refractive index N_(c) =1.408) lower than therefractive index (N_(a)) of the polymer A was used as a refractive indexmodifier C. These two components were mixed (mixing ratio=4:1) toprepare an initial solution. The mixture solution was dissolved in asolvent (tetrahydrofuran: THF) in a concentration of about 30 wt % andthe resultant solution was poured into a supply tank 14.

Further, a hollow cylindrical member 11 (refractive index: 1.410) wasprepared by using a mixture solution having the same composition as thatof the above solution, as a main component. On the other hand, the abovepolymer A was dissolved in THF (concentration: about 30 wt. %) to obtaina solution, and the resultant solution was stored in a refractive indexmodifier tank 15.

The hollow cylindrical member 11 thus obtained was mounted to an unshownrotating device and the vapor of a raw material was directly suppliedfrom the supply tank 14 onto the inner surface of the hollow cylindricalmember 11 through a supply pipe 19 to be deposited thereon.

In the above deposition, the raw material A was gradually fed from therefractive index modifier tank 15 into the supply tank 14, so that themixing ratio of the raw material A was increased at every depositionoperation to gradually increase the refractive index of the depositionlayer based on the vapor. As a result, there was obtained a plasticoptical fiber preform 21 (FIG. 4) having a GI-type refractive indexdistribution in which the refractive index was gradually decreased fromthe center of the preform toward the outer periphery thereof (as shownin FIG. 5).

<Evaluation of transmission property>

Each of the preforms obtained in above Examples 1 and 2 was drawn undermelting by a conventional drawing process to prepare an all-plasticoptical fiber (APF) having a diameter of 1 mm and a length of 100 m.Each of these fibers was evaluated with respect to transmission loss andband at a wavelength of 0.658 μm. The thus obtained evaluation resultsare shown in the following table (Table 1).

In this evaluation, a laser diode (LD) having a wavelength of 0.658 μmwas used as a light source for measurement of both the transmission lossand the band. Further, the band was measured by use of an FFT typeoptical oscilloscope (manufactured by Hamamatsu Photonics k.k.) whilepulses with a half-value width of 60 psec were generated in the lightsource.

                  TABLE 1                                                         ______________________________________                                                       <Example 1>                                                                             <Example 2>                                          ______________________________________                                        Transmission loss (dB/km)                                                                      100         120                                              Transmission band (MHz · km)                                                          800         800                                              ______________________________________                                    

EXAMPLE 3 (Production of fiber preform by inside-surface CVD)

Referring to FIG. 1, a glass cylindrical member 11 having an innerdiameter of 50 mm and an outer diameter of 52 mm was prepared, and aglass pipe having an inner diameter of 4 mm and an outer diameter of 5mm was fixed at a position corresponding to the center of thecylindrical member. The glass pipe used herein had holes bored thereinand having a diameter of 1 mm at intervals of 5 mm.

First, methyl methacrylate (MMA) and an initiator (benzil methyl ketal)were heated to 80° C. and the vapor of these materials was introducedinto the above-mentioned glass pipe. The vapor ejected from the glasspipe was attached to the inner surface of the above glass cylindricalmember. The glass cylindrical member was rotated at a rotational speedof about one rotation per minute.

When the vapor attached to the inner surface of the glass cylindricalmember was heated at 100° C. by use of a ring heater, a cladding layer(thickness: 5 mm) was formed on the basis of the polymerization of theabove-mentioned methyl methacrylate.

With respect to the formation of a core, methyl methacrylate and aninitiator were introduced into the glass pipe provided with the claddinglayer formed above, and were subjected to polymerization under the sameconditions as those for the formation of the above cladding layer,except for controlling the mixing ratio of a dopant (butyl benzylphthalate; BBP). At the time of the control of the mixing ratio of thedopant, a solution of the dopant (concentration: about 30 wt. %) wasplaced in a dopant vessel 15, and was supplied at 10 ml/min from thevessel to the supply tank 14 in the formation of the above core.

After the glass pipe 11 was removed from the deposition product thusobtained, the resultant product was subjected to collapse under heatingat 100° C., thereby to obtain a base material (preform) having arefractive index distribution as shown in a graph of FIG. 21. In FIG.21, the abscissa (r/Rp) represents a distance from the core center (interms of a relative value with respect to "25 mm"=1 (one) unit) and theordinate (n-n₀) represents a relative refractive index difference withrespect to the refractive index of the cladding. In the graph of FIG.21, the curve A shows the refractive index distribution of a preform inwhich a mixing ratio in the neighborhood of the core center isMMA/BBP=7/1, the curve B shows the refractive index distribution of apreform in which the mixing ratio in the neighborhood of the core centeris MMA/BBP=6/1, and the curve C shows a refractive index distribution ofa preform in which the mixing ratio in the neighborhood of the-corecenter is MMA/BBP=5/1.

EXAMPLE 4

Referring to FIG. 7, polymethyl methacrylate (PMMA; refractive indexN_(a) =1.490) was used as a transparent polymer A to form a hollowcylindrical member 31 mainly comprising the polymer A.

The polymer A (PMMA) was dissolved in a solvent (tetrahydrofuran: THF)in a predetermined ratio (concentration: about 30 wt. %) and theresultant solution was poured into a supply tank 34. On the other hand,butyl benzyl phthalate as a refractive index modifier B having arefractive index (refractive index N_(b) =1.536) higher than that of thepolymer A was dissolved in THF (concentration: about 30 wt. %) to obtaina solution, and the resultant solution was stored in a refractive indexmodifier tank 35.

The hollow cylindrical member 31 obtained by the above procedure wasmounted to an unshown rotating device and the solution of raw materialfrom the supply tank 14 was sprayed uniformly on the inner surface ofthe cylindrical member along the axis direction thereof, by use of anozzle 32.

In the above spraying operation, the refractive index modifier B wasgradually fed (flow rate: about 10 ml/min) from the refractive indexmodifier tank 35 into the supply tank 34, so that the mixing ratio ofthe refractive index modifier B was increased at every sprayingoperation to gradually increase the refractive index of the sprayingsolution. When such an operation was repeated 100 times, there wasobtained a plastic optical fiber preform 21 (FIG. 4) having a GI-typerefractive index distribution in which the refractive index wasgradually decreased from the center of the preform toward the outerperiphery thereof (as shown in FIG. 5).

EXAMPLE 5

Referring to FIG. 7, polymethyl methacrylate (PMMA; refractive indexN_(a) =1.492) was used as a transparent polymer A, and hexyl acetatehaving a refractive index (refractive index N_(c) =1.408) lower than therefractive index (N_(a)) of the polymer A was used as a refractive indexmodifier C. These two components were mixed (mixing ratio=4:1) toprepare an initial solution. Thus obtained mixture solution wasdissolved in a solvent (tetrahydrofuran: THF) in a concentration ofabout 30 wt. % and the resultant solution was poured into a supply tank34.

Further, a hollow cylindrical member (refractive index: 1.410) 31 wasprepared by using a mixture solution having the same composition as thatof the mixture solution described above, as a main component. On theother hand, the above polymer A was dissolved in THF (concentration:about 30 wt. %) to obtain a solution, and the resultant solution wasstored in a refractive index modifier tank 35.

The hollow cylindrical member 31 thus obtained was mounted to an unshownrotating device and the raw material from the supply tank 34 was sprayeduniformly onto the inside surface of the hollow cylindrical member 31along the axis direction thereof, by use of the nozzle 32.

At the time of the spraying operation, the raw material A was graduallyfed from the refractive index modifier tank 35 into the supply tank 34,so that the mixing ratio of the raw material A was increased at everyspraying operation to gradually increase the refractive index of thespraying solution. When this operation was repeated 100 times, there wasobtained a plastic optical fiber preform 21 (FIG. 4) having a GI-typerefractive index distribution in which the refractive index wasgradually decreased from the center of the preform toward the outerperiphery thereof (as shown in FIG. 5).

<Evaluation of transmission property>

Each of the preforms obtained in above Examples 4 and 5 was drawn undermelting by a conventional drawing process to prepare an all-plasticoptical fiber (APF) having a diameter of 1 mm and a length of 100 m.Each of these fibers was evaluated with respect to transmission loss andband at a wavelength of 0.658 μm. The thus obtained evaluation resultsare shown in the following table (Table 2).

In this evaluation, a laser diode (LD) having a wavelength of 0.658 μmwas used as a light source for measurement of both the transmission lossand the band. Further, the band was measured by use of an FFT typeoptical oscilloscope (manufactured by Hamamatsu Photonics k.k.) whilepulses with a half-value width of 60 psec were generated in the lightsource.

                  TABLE 2                                                         ______________________________________                                                       <Example 4>                                                                             <Example 5>                                          ______________________________________                                        Transmission loss (dB/km)                                                                      150         160                                              Transmission band (MHz · km)                                                          700         800                                              ______________________________________                                    

EXAMPLE 6 (Production of fiber preform by coating process)

Referring to FIG. 8, a glass cylindrical member 31 having an innerdiameter of 50 mm and an outer diameter of 52 mm was prepared, and aglass pipe 32a having an inner diameter of 4 mm and an outer diameter of5 mm was fixed at a position corresponding to the center of thecylindrical member. The glass pipe used herein had holes bored thereinand-having a diameter of 5 mm at intervals of 5 mm.

First, a THF solution (concentration: about 30 wt. %) of polymethylmethacrylate (PMMA) was placed in a supply tank 34, and the solution wassupplied into the glass pipe 32a (flow rate: about 10 ml/min). Thesolution was uniformly sprayed from the pipe 32a onto the inside surfaceof the glass cylindrical member 31 along the axis direction thereof. Theglass cylindrical member was rotated at a rotational speed of about onerotation per minute.

When the solution attached to the inner surface of the glass cylindricalmember was heated at 100° C. by means of a heater 33, a cladding layer(thickness: 5 mm) was formed on the basis of the spraying of the abovesolution of polymer A.

With respect to the formation of a core, the above methyl methacrylatesolution was introduced into the glass pipe 31 provided with thecladding layer formed above, and was subjected to spraying under thesame conditions as those for the formation of the above cladding layer,except for controlling the mixing ratio of a dopant (butyl benzylphthalate: BBP). At the time of the control of the mixing ratio of thedopant, a solution of the dopant (concentration: about 30 wt. %) wasplaced in a dopant vessel 35, and was supplied at about 10 ml/min fromthe vessel to the supply tank 34 in the formation of the above core.

After the glass pipe 31 was removed from the deposition product thusobtained, the resultant product was subjected to collapse under heatingat 100° C., thereby to obtain a preform. In the thus obtained preforms,the mixing ratio in the neighborhood of the core center was MMA/BBP=7/1,6/1 and 5/1, respectively. Each of these preforms had a refractive indexdistribution which was substantially comparable to that shown in theabove-mentioned graph of FIG. 21.

EXAMPLE 7

By use of a fiber drawing apparatus as shown in FIG. 9, a plasticoptical fiber preform 40 provided with a GI-type refractive indexdistribution was subjected to a fiber drawing operation.

More specifically, the above preform 40 was inserted into a fiberdrawing furnace 41 in which the temperature in the core tube was set to240° C., and was subjected to fiber drawing at a linear velocity of 2m/min so as to provide a center value of the resultant outer diameter of650 μm. The drawing tension used herein was 20 g.

When the tensile strength of the thus prepared fiber 46 was measured, itwas found to be 2.3 Kg/mm². When a length of 1 m of the fiber was woundup with a tension of 50 g around a mandrel having a diameter of 10 mm,the period of time until the breakage thereof was 10 days.

EXAMPLE 8

Referring to FIG. 9, a plastic optical fiber preform 40 provided with aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 230° C., and was subjected to fiber drawing at a linear velocityof 2 m/min so as to provide a center value of the resultant outerdiameter of 650 μm. The drawing tension used herein was 40 g.

When the tensile strength of the thus prepared fiber 46 was measured, itwas found to be 2.4 Kg/mm². When a length of 1 m of the fiber was woundup with a tension of 50 g around a mandrel having a diameter of 10 mm,the period of time until the breakage thereof was 12 days.

EXAMPLE 9

Referring to FIG. 9, a plastic optical fiber preform 40 provided with aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 250° C., and was subjected to fiber drawing at a linear velocityof 2 m/min so as to provide a center value of the resultant outerdiameter of 650 μm. The drawing tension used herein was 15 g.

When the tensile strength of the thus prepared fiber 46 was measured, itwas found to be 2.3 Kg/mm². When a length of 1 m of the fiber was woundup with a tension of 50 g around a mandrel having a diameter of 10 mm,the period of time until the breakage thereof was 8 days.

COMPARATIVE EXAMPLE 1

Referring to FIG. 9, a plastic optical fiber preform 40 provided with aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 260° C., and was subjected to fiber drawing at a linear velocityof 2 m/min so as to provide a center value of the resultant outerdiameter of 650 μm. The drawing tension used herein was 8 g.

When the tensile strength of the thus prepared fiber 46 was measured, itwas found to be 1.5 Kg/mm². When a length of 1 m of the fiber was woundup with a tension of 50 g around a mandrel having a diameter of 10 mm,the period of time until the breakage thereof was 10 hours.

COMPARATIVE EXAMPLE 2

Referring to FIG. 9, a plastic optical fiber preform 40 provided with aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 260° C., and was subjected to fiber drawing at a linear velocityof 1.5 m/min so as to provide a center value of the resultant outerdiameter of 650 μm. The drawing tension used herein was 6 g.

When the tensile strength of the thus prepared fiber 46 was measured, itwas found to be 1.3 Kg/mm². When a length of 1 m of the fiber was woundup with a tension of 50 g around a mandrel having a diameter of 10 mm,the period of time until the breakage thereof was 8 hours.

COMPARATIVE EXAMPLE 3

Referring to FIG. 9, a plastic optical fiber preform 40 provided with aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 270° C., and was subjected to fiber drawing at a linear velocityof 2 m/min so as to provide a center value of the resultant outerdiameter of 650 μm. The drawing tension used herein was 5 g.

When the tensile strength of the thus prepared fiber 46 was measured, itwas found to be 1.0 Kg/mm². When a length of 1 m of the fiber was woundup with a tension of 50 g around a mandrel having a diameter of 10 mm,the period of time until the breakage thereof was 3 hours.

COMPARATIVE EXAMPLE 4

Referring to FIG. 9, a plastic optical fiber preform 40 provided with aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 275° C., and was subjected to fiber drawing at a linear velocityof 2 m/min so as to provide a center value of the resultant outerdiameter of 1100 μm. The drawing tension used herein was 5 g.

When the tensile strength of the thus prepared fiber 46 was measured, itwas found to be 2.2 Kg/mm². When a length of 1 m of the fiber was woundup with a tension of 50 g around a mandrel having a diameter of 10 mm,the period of time until the breakage thereof was 10 days, andsubstantially no decrease in the strength was observed.

EXAMPLE 10

Referring to FIG. 9, a preform for plastic optical fiber 40 having aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the heater 42 comprised a carbonheater having a length of healing zone of 10 mm and the temperature inthe core tube was set at 220° C.

When the above preform was subjected to fiber drawing at a linearvelocity of 2 m/min so as to provide a center value of the resultantouter diameter of 650 μm. The resultant fluctuation in the outerdiameter was ±30 μm. At the time of the above fiber drawing operation,the drawing tension was set to 70 g.

When the transmission loss of the thus prepared GI-type plastic opticalfiber 46 was measured, it was found to be 200 dB/km at a wavelength of650 nm.

Then, the above fiber was subjected to a deterioration treatment at 80°C. for one day (24 hours), and then the shrinkage retention (retentionin shrinkage) and transmission loss thereof were measured. As a result,they were found to be 99% and 210 dB/km, respectively. In other words,substantially no shrinkage due to heat was observed, and the fluctuationin the transmission loss was little. The results of the measurement ofthe transmission loss are shown in a graph of FIG. 22.

EXAMPLE 11

Referring to FIG. 9, a preform for plastic optical fiber 40 having aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the heater 42 comprised a carbonheater having a length of heating zone of 10 mm and the temperature inthe core tube was set at 220° C.

When the above preform was subjected to fiber drawing at a linearvelocity of 3 m/min so as to provide a center value of the resultantouter diameter of 650 μm. The resultant fluctuation in the outerdiameter was ±30 μm. At the time of the above fiber drawing operation,the drawing tension was set to 85 g.

When the transmission loss of the thus prepared GI-type plastic opticalfiber 46 was measured, it was found to be 210 dB/km at a wavelength of650 nm.

Then, the above fiber was subjected to a deterioration treatment at 80°C. for one day, and then the shrinkage retention and transmission lossthereof were measured. As a result, they were found to be 99% and 190dB/km, respectively. In other words, substantially no shrinkage due toheat was observed, and the fluctuation in the transmission loss waslittle. The results of the measurement of the transmission loss areshown in a graph of FIG. 22.

EXAMPLE 12

Referring to FIG. 9, a preform for plastic optical fiber 40 having aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the heater 42 comprised a carbonheater having a length of heating zone of 10 mm and the temperature inthe core tube was set at 230° C.

When the above preform was subjected to fiber drawing at a linearvelocity of 2 m/min so as to provide a center value of the resultantouter diameter of 650 μm. The resultant fluctuation in the outerdiameter was ±30 μm. At the time of the above fiber drawing operation,the drawing tension was set to 50 g.

When the transmission loss of the thus prepared GI-type plastic opticalfiber 46 was measured, it was found to be 220 dB/km at a wavelength of650 nm.

Then, the above fiber was subjected to a deterioration treatment at 80°C. for one day, and then the shrinkage retention and transmission lossthereof were measured. As a result, they were found to be 98% and 210dB/km, respectively. In other words, substantially no shrinkage due toheat was observed, and the fluctuation in the transmission loss waslittle. The results of the measurement of the transmission loss areshown in a graph of FIG. 22.

EXAMPLE 13

Referring to FIG. 9, a preform for plastic optical fiber 40 having aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the heater 42 comprised a heatercomprising carbon resistance (carbon heater) having a length of heatingzone of 10 mm and the temperature in the core tuber was set at 210° C.

When the above preform was subjected to fiber drawing at a linearvelocity of 2 m/min so as to provide a center value of the resultantouter diameter of 650 μm. The resultant fluctuation in the outerdiameter was ±30 μm. At the time of the above fiber drawing operation,the drawing tension was set to 100 g.

When the transmission loss of the thus prepared GI-type plastic opticalfiber 46 was measured, it was found to be 200 dB/km at a wavelength of650 nm.

Then, the above fiber was subjected to a deterioration treatment at 80°C. for one day, and then the shrinkage retention and transmission lossthereof were measured. As a result, they were found to be 97% and 230dB/km, respectively. In other words, substantially no shrinkage due toheat was observed, and the fluctuation in the transmission loss waslittle. The results of the measurement of the transmission loss areshown in a graph of FIG. 22.

COMPARATIVE EXAMPLE 5

Referring to FIG. 9, a preform for plastic optical fiber 40 having aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 200° C.

When the above preform was subjected to fiber drawing at a linearvelocity of 2 m/min so as to provide a center value of the resultantouter diameter of 650 μm. At the time of the above fiber drawingoperation, the drawing tension was set to 120 g.

When the transmission loss of the thus prepared GI-type plastic opticalfiber 46 was measured, it was found to be 200 dB/km at a wavelength of650 nm.

Then, the above fiber was subjected to a deterioration treatment at 80°C. for one day, and then the shrinkage retention and transmission lossthereof were measured. As a result, they were found to be 90% and 300dB/km, respectively. In other words, a somewhat larger shrinkage due toheat was observed, and the fluctuation in the transmission loss was alsolarge. The results of the measurement of the transmission loss are shownin a graph of FIG. 22.

COMPARATIVE EXAMPLE 6

Referring to FIG. 9, a preform for plastic optical fiber 40 having aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 200° C.

When the above preform was subjected to fiber drawing at a linearvelocity of 3 m/min so as to provide a center value of the resultantouter diameter of 650 μm. At the time of the above fiber drawingoperation, the drawing tension was set to 150 g.

When the transmission loss of the thus prepared GI-type plastic opticalfiber 46 was measured, it was found to be 230 dB/km at a wavelength of650 nm.

Then, the above fiber was subjected to a deterioration treatment at 80°C. for one day, and then the shrinkage retention and transmission lossthereof were measured. As a result, they were found to be 90% and 350dB/km, respectively. In other words, a somewhat larger shrinkage due toheat was observed, and the fluctuation in the transmission loss was alsolarge. The results of the measurement of the transmission loss are shownin a graph of FIG. 22.

COMPARATIVE EXAMPLE 7

Referring to FIG. 9, a preform for plastic optical fiber 40 having aGI-type refractive index distribution was provided and was inserted intoa fiber drawing furnace 41 in which the temperature in the core tube wasset to 220° C.

When the above preform was subjected to fiber drawing at a linearvelocity of 4 m/min so as to provide a center value of the resultantouter diameter of 650 μm. At the time of the above fiber drawingoperation, the drawing tension was set to 130 g.

When the transmission loss of the thus prepared GI-type plastic opticalfiber 46 was measured, it was found to be 240 dB/km at a wavelength of650 nm.

Then, the above fiber was subjected to a deterioration treatment at 80°C. for one day, and then the shrinkage retention and transmission lossthereof were measured. As a result, they were found to be 80% and 400dB/km, respectively. In other words, a somewhat larger shrinkage due toheat was observed, and the fluctuation in the transmission loss was alsolarge. The results of the measurement of the transmission loss are shownin a graph of FIG. 22.

As shown in the graph of FIG. 22 described above, in the drawing of aplastic optical fiber conducted in the above Examples 10-13, the fiberdrawing was conducted while the fiber drawing tension was set to 100 gor less, whereby an optical fiber having a smaller shrinkage retentionand a smaller fluctuation in the transmission loss could be obtained, ascompared with the fiber obtained in Comparative Examples 5-7. In otherwords, in these Examples, it was found that the shrinkage after heatdeterioration was controlled and the increase in transmission loss wasreduced.

INDUSTRIAL APPLICABILITY

As described hereinabove, according to the present invention, it ispossible to easily accomplish the control of a refractive indexdistribution which has been troublesome in the prior art, and to providea GI-type plastic optical fiber preform with good reproducibility.

In the present invention, in a case where the tension for drawing is setto a value of 10 g or more at the time of the production of a plasticoptical fiber based on drawing (particularly, when the outer diameter ofthe resultant fiber is 1000 μm or less), it is possible to furtherimprove the resultant fiber in mechanical strength and long-termreliability.

Further, in the present invention, in a case where the tension fordrawing is set to a value of 100 g or less at the time of the productionof a plastic optical fiber based on drawing, the shrinkage of theresultant fiber can be suppressed and the increase in the transmissionloss can be suppressed even after the heat deterioration thereof.

Furthermore, according to the present invention, the following effectsmay be achieved even in the case of a plastic optical fiber preformhaving a jacket layer.

(1) In a case where a fiber preform comprising a plastic material isprepared by using a substance having the same material quality andhaving a high purity for all portions of the fiber preform, theproduction cost inevitably becomes higher. However, in a case where theneighborhood of the circumference of the cladding layer which does notcontribute to light transmission is constituted by using a jacket layerhaving a lower purity, the resultant production cost may be decreased.For example, even when an optical fiber preform having the samespecification or standard is produced, a part thereof can be formed byusing a material having a lower purity, thereby to further reduce theproduction cost.

(2) Further, in an embodiment in which a functional material such asanti-oxidant, light absorbing agent, and light scattering agent isincorporated in the above jacket layer, it is easy to obtain a plasticoptical fiber having any of such various functions.

(3) In an embodiment in which a refractive index modifier isincorporated in the same manner as described above, it is easy to obtaina GI-type plastic optical fiber preform in the same manner as in theformation of the above jacket layer.

We claim:
 1. A process for producing a preform for a plastic opticalfiber having a refractive index distribution in which the refractiveindex is gradually radially decreased from a center of the preformtoward an outer periphery thereof, by depositing a deposition layercomprising a polymer A having a refractive index of N_(a) and arefractive index modifier having a refractive index different from thatof the polymer A onto an inner surface of a hollow cylindrical memberrotating about an axis thereof, by use of vapor-phase deposition basedon a Chemical Vapor Deposition (CVD) process,wherein a mixing ratiobetween the polymer A and the refractive index modifier for forming thedeposition layer by said CVD process is changed to gradually increasethe refractive index of the deposition layer from said outer peripheryof said preform to said center of said preform.
 2. A process forproducing a preform for a plastic optical fiber according to claim 1,wherein the refractive index modifier comprises a refractive indexmodifier B having a refractive index (N_(b)) higher than that of thepolymer A, and the mixing ratio of the refractive index modifier B topolymer A is gradually increased.
 3. A process for producing a preformfor a plastic optical fiber according to claim 1, wherein the refractiveindex modifier comprises a refractive index modifier C having arefractive index (N_(c)) lower than that of the polymer A, and themixing ratio of the refractive index modifier C to polymer A isgradually decreased.
 4. A process for producing a preform for a plasticoptical fiber according to claim 1, wherein the hollow cylindricalmember is comprised of a polymer B whereby the monomer units of polymerB are substantially the same as the monomer units of polymer A, andwherein the monomer units of polymer B have a purity lower than that ofthe monomer units of polymer A present in the material constituting thedeposition layer.
 5. A process for producing a preform for plasticoptical fiber according to claim 4, wherein the hollow cylindricalmember contains a functional material.
 6. A process for producing apreform for plastic optical fiber according to claim 5, wherein thefunctional material comprises at least one material selected from thegroup consisting of an anti-oxidant, a light absorbing agent, and alight scattering agent.
 7. A process for producing a preform for aplastic optical fiber according to claim 1, wherein said refractiveindex modifier is a non-polymerizable material.
 8. A process forproducing a preform for a plastic optical fiber according to claim 1,wherein the absolute value of the difference between the refractiveindices of said polymer A and said refractive index modifier is greaterthan 0.02.
 9. A process for producing a preform for a plastic opticalfiber having a refractive index distribution in which the refractiveindex is gradually radially decreased from a center of the preformtoward an outer periphery thereof, by depositing a deposition layercomprising a polymer A having a refractive index of N_(a) and arefractive index modifier having a refractive index different from thatof the polymer A onto an inner surface of a hollow cylindrical memberrotating about an axis thereof, by use of a coating process,wherein amixing ratio between the polymer A and the refractive index modifier forforming the deposition layer by said coating process is changed togradually increase the refractive index of the deposition layer fromsaid outer periphery of said preform to said center of said preform. 10.A process for producing a preform for plastic optical fiber according toclaim 9, wherein the coating process includes a spraying process.
 11. Aprocess for producing a preform for a plastic optical fiber according toclaim 9, wherein the refractive index modifier comprises a refractiveindex modifier B having a refractive index (N_(b)) higher than that ofthe polymer A, and the mixing ratio of the refractive index modifier Bto polymer A is gradually increased.
 12. A process for producing apreform for a plastic optical fiber according to claim 9, wherein therefractive index modifier comprises a refractive index modifier C havinga refractive index (N_(c)) lower than that of the polymer A, and themixing ratio of the refractive index modifier C to polymer A isgradually decreased.
 13. A process for producing a preform for a plasticoptical fiber according to claim 9, wherein the hollow cylindricalmember is comprised of a polymer B whereby the monomer units of polymerB are substantially the same as the monomer units of polymer A, andwherein the monomer units of polymer B have a purity lower than that ofthe monomer units of polymer A present in the material constituting thedeposition layer.
 14. A process for producing a preform for a plasticoptical fiber according to claim 9, wherein said refractive indexmodifier is a non-polymerizable material.
 15. A process for producing apreform for a plastic optical fiber according to claim 9, wherein theabsolute value of the difference between the refractive indices of saidpolymer A and said refractive index modifier is greater than 0.02.